Mechanics
Classical mechanics  
Newton's Second Law 

History of ...


Quantum mechanics  
Uncertainty principle 

Introduction to... Mathematical formulation of...


Mechanics (from the Greek term Μηχανική) is a branch of physics involving study of the movement of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment. This discipline, which has its roots in several ancient civilizations, is now subdivided into two main branches: classical mechanics and quantum mechanics.
During the early modern period, scientists such as Galileo, Johannes Kepler, and especially Isaac Newton, laid the foundations for what is now known as classical mechanics. The foundations of quantum mechanics were established during the first half of the twentieth century by Max Planck, Werner Heisenberg, Louis de Broglie, Albert Einstein, Niels Bohr, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Wolfgang Pauli and others. Quantum mechanics is now considered a foundationallevel theory that encompasses and supersedes classical mechanics. However, classical mechanics is useful for calculations of macroscopic processes, while quantum mechanics helps explain and predict processes at the molecular, atomic, and subatomic levels.
Studies in mechanics have made vital contributions to various fields of engineering. They include mechanical engineering, aerospace engineering, civil engineering, structural engineering, materials engineering, and biomedical engineering. Thus, knowledge of mechanics has led to many practical applications.
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Significance
Mechanics is the original discipline of physics and was formerly part of "natural philosophy," dealing with forces and motion in the macroscopic world as perceived by the human eye. This discipline has developed into a huge body of knowledge about important aspects of the natural world. Modern mechanics encompasses the movement of all matter in the universe under the four fundamental interactions (or forces): gravity, the strong and weak interactions, and the electromagnetic interaction.
Mechanics also constitutes a central part of technology, the application of physical knowledge for human purposes. In this sense, the discipline is often known as engineering or applied mechanics, and it is used to design and analyze the behavior of structures, mechanisms, and machines. Important aspects of the fields of mechanical engineering, aerospace engineering, civil engineering, structural engineering, materials engineering, biomedical engineering and biomechanics were spawned from the study of mechanics.
Classical versus quantum mechanics
The major division of the discipline of mechanics is one that separates classical mechanics from quantum mechanics. Historically, classical mechanics came first, while quantum mechanics is a comparatively recent formulation. Classical mechanics originated with Isaac Newton's Laws of motion in Principia Mathematica, while quantum mechanics did not appear until 1900. Both are commonly held to constitute the most certain knowledge that exists about physical nature. Classical mechanics has especially often been viewed as a model for other socalled exact sciences. Essential in this respect is the relentless use of mathematics in theories, as well as the decisive role played by experiment in generating and testing them.
Quantum mechanics is of a wider scope, as it encompasses classical mechanics as a subdiscipline that is applicable under certain restricted circumstances. According to the correspondence principle, there is no contradiction or conflict between the two subjects, each simply pertains to specific situations. Quantum mechanics has superseded classical mechanics at the foundational level and is indispensable for the explanation and prediction of processes at molecular, atomic, and subatomic levels. However, for macroscopic processes, classical mechanics is able to solve problems that are unmanageably difficult in quantum mechanics and hence remains useful and well used.
Einsteinian versus Newtonian physics
Analogous to the quantum reformation of classical mechanics, Einstein's general and special theories of relativity have expanded the scope of mechanics beyond the mechanics of Newton and Galileo, and made fundamental corrections to them, that become significant and even dominant as speeds of material objects approach the speed of light, which cannot be exceeded.
Relativistic corrections are also needed for quantum mechanics, although relativity has not been fully integrated with it yet. This is one of the hurdles that has to be overcome in developing a Grand Unified Theory.
Types of mechanical bodies
The oftenused term body needs to stand for a wide assortment of objects, including particles, projectiles, spacecraft, stars, parts of machinery, parts of solids, parts of fluids (gases and liquids), and so forth.
Other distinctions between the various subdisciplines of mechanics, concern the nature of the bodies being described. Particles are bodies with littleknown internal structure, treated as mathematical points in classical mechanics. Rigid bodies have size and shape, but retain a simplicity close to that of the particle, adding just a few socalled degrees of freedom, such as orientation in space.
Otherwise, bodies may be semirigid, that is, elastic, or nonrigid, that is, fluid. These subjects have both classical and quantum divisions of study.
For instance, the motion of a spacecraft, regarding its orbit and attitude (rotation), is described by the relativistic theory of classical mechanics. Analogous motions of an atomic nucleus are described by quantum mechanics.
Subdisciplines of mechanics
The following two lists indicate various subjects that are studied under classical mechanics and quantum mechanics.
Classical mechanics
The following areas are included as part of the field of classical mechanics:
 Newtonian mechanics, involves the original theory of motion (kinematics) and forces (dynamics)
 Lagrangian mechanics, a theoretical formalism, based on the principle of conservation of energy
 Hamiltonian mechanics, another theoretical formalism, based on the principle of the least action
 Celestial mechanics, the motion of heavenly bodies, such as planets, comets, stars, and galaxies
 Astrodynamics, for the navigation of spacecraft and similar objects
 Solid mechanics, involving study of elasticity and the properties of (semi)rigid bodies
 Acoustics, dealing with sound (or density variation propagation) in solids, fluids, and gases.
 Statics, dealing with semirigid bodies in mechanical equilibrium
 Fluid mechanics, or the study of the motion of fluids
 Soil mechanics, or the study of the mechanical behavior of soils
 Continuum mechanics, involving the mechanics of continua (both solid and fluid)
 Hydraulics, dealing with the mechanical properties of liquids
 Fluid statics, dealing with liquids in equilibrium
 Applied / Engineering mechanics, for technological applications
 Biomechanics, studying biological materials
 Biophysics, studying the physical processes in living organisms
 Statistical mechanics, dealing with assemblies of particles too large to be described in a deterministic way
 Relativistic or Einsteinian mechanics, dealing with universal gravitation
Quantum mechanics
The following areas are categorized as being part of the field of quantum mechanics:
 Particle physics, related to the motion, structure, and reactions of particles
 Nuclear physics, related to the motion, structure, and reactions of atomic nuclei
 Condensed matter physics, involving the study of quantum gases, solids, and liquids
 Quantum statistical mechanics, dealing with large assemblies of particles
In addition to the above areas, there is the "theory of fields," which constitutes a separate discipline in physics, formally treated as distinct from mechanics, whether classical fields or quantum fields. But in actual practice, subjects belonging to mechanics and fields are closely interwoven. Thus, for instance, forces that act on particles are frequently derived from fields (electromagnetic or gravitational), and particles generate fields by acting as sources. In fact, in quantum mechanics, particles themselves are fields, as described theoretically by the wave function.
See also
 Albert Einstein
 Engineering
 Galileo
 Isaac Newton
 Johannes Kepler
 Kinematics
 Kinetics
 Machine
 Max Planck
 Quantum mechanics
References
 Beer, Ferdinand Pierre, E. Russell Johnston, and John T. DeWolf. 2006. Mechanics of Materials. Boston: McGrawHill Higher Education. ISBN 9780073107950.
 Byron, Frederick W., and Robert W. Fuller. [1969] 1992. Mathematics of Classical and Quantum Physics. reprint ed., New York: Dover Publications. ISBN 048667164X.
 Griffiths, David J. 2005. Introduction to Quantum Mechanics, 2nd ed. Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 9780131118928.
 Hibbeler, R. C. 2007. Engineering Mechanics: Dynamics. Upper Saddle River, NJ: Pearson/Prentice Hall. ISBN 9780132215046.
 Hibbeler, R. C. 2008. Mechanics of Materials, 7th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 9780132209915.
 Messiah, Albert. [1958] 1999. Quantum Mechanics. reprint ed., Mineola, NY: Dover Publications. ISBN 0486409244.
 Munson, Bruce Roy, Donald F. Young, and T. H. Okiishi. 2006. Fundamentals of Fluid Mechanics. Hoboken, NJ: J. Wiley & Sons. ISBN 9780471675822.
 Taylor, John R. 2005. Classical Mechanics. Sausalito, CA: University Science Books. ISBN 9781891389221.
External links
 iMechanica: web of mechanics and mechanicians Retrieved July 25, 2008.
 Physclips: Mechanics with animations and video clips University of New South Wales. Retrieved July 25, 2008.
 U.S. National Committee on Theoretical and Applied Mechanics. Retrieved July 25, 2008.
 The Physics Department  Mechanics. Retrieved July 25, 2008.
 Mechanics. Retrieved July 25, 2008.
 American Society of Mechanical Engineers. Retrieved July 25, 2008.
 American Physical Society. Retrieved July 25, 2008.
 Institution of Mechanical Engineers. Retrieved July 25, 2008.
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